Multifrequency directive antenna array



July 9, 1940- A. ALFoRD MULTIFREQUENCVY DIRECTIVE ANTENNA ARRAY Filed Deo. 10, 1937 FIG. 3.

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54,1/5 INVENTOR 4A/.DREW AL fr0/95 MEA/VS ATTORNEY Patented July 9, 1940 MULTIFREQUENCY DIRECTIVE ANTENNA f l ARRAY j Andrew Alford, San Mateo, Calif., assigner to Mackay Radio & Telegraph Company, New York, N. Y., a corporation of Delaware Application December 10, 1937, Serial No. 179,082

' 1o claims. ((1250-11) Y This invention relates to directional antenna systems and pertains more particularly to directional antenna systems Which are adapted to transmit or receive at a plurality of different frequencies.

With respect to the directional operation the present invention relates to both the gain and the front to back discrimination. In other Words the invention relates both to the general concentration of the radiation or reception pattern into a narrow lobe with no secondary lobes or with only weak secondary lobes and also to the elimination of the backwardly directed lobe of the pattern. In some aspects, however, the present invention is particularly directed to the last mentioned feature, namely, the -reducton of the backward radiation.

Generally, it is an object ofl my invention to provide a system of antennae which comprises a plurality of groups each including a plurality of antennae, and to organize these groups and the antennae within each group so as to completely or substantially neutralize back radiation from the system at each of a number of frequencies.

More particularly, it is an object of my invention to provide a system of four antennae',` consisting of two pairs of antenna which is adapted to completely neutralize back radiation at each of two predetermined frequenciesv and shall also substantially neutralize back radiation at one or more other frequencies.

'In accordance with the preferred forml of my invention I provide two pairs of'radiating and reflecting antennae, the spacing of the antennae forming each pair being such that for a iirst frequency the transmission of each pair is unidirectional, and the spacing between the two pairs of antennae being such that for a second frequency the two pairs transmit unidirectionally. The antennae are preferably of the V type.

The above outlined arrangement of four antennae can be adjusted to completely neutralize back radiation with respect to two predetermined frequencies. Ordinarily, moreover, for additional frequencies falling in the neighborhood of each of these two predetermined frequencies, the back radiation will also be substantially neutralized. No simple rule can be laid down to dene exactly how close the additional frequencies must be to the primary predetermined frequencies in order to obtain such substantial neutralization for these addition frequencies. The breadth of the regions of substantial neutralization of back radiation depends not only upon the magnitude of the spacing between the pairs of antennae and between the two antennae of eachpair, but also if' upon the mutual proximity in the frequency spec; trum of the two predetermined frequencies. Ordinarily it will be found that if the two-,primary frequencies are selected fairly close to each other and the spacing between the antenna pairs, as well as ,the spacings between the individual an` tennae in each pair, are takento be less than one wavelength, thewhole band of frequencies lying between the two chosen frequencies yand even extending slightly .above and below these, will be capable of transmissionwithout excessive back radiation. 1 y

By means of a system constructed in accordance with my inventionit is, therefore, possible to unidirectionally transmit over an antenna system having two pairs of antennae, not only'the two primary frequencies for which the spacings of the several antennae are selected to cancel back radiation', but also one or several additional frequencies Afalling between the two primary frequenciesjor being only slightly highery or lower than these frequencies.

The above mentioned and further objects and advantages of my invention land the manner of obtaining them will be more fully explainedin -ff* the following description taken in conjunction with .the vaccompanying drawing.

In the drawing,

Fig. 1 illustrates diagrammaticallyan antenna system constructed in accordance with my invention,

Fig. 2 .is a diagrammatic View of a pair of antennae, used in describing myinvention, and

Fig. 3 is a set of curves showing the ratios of backward to forward radiations from antenna tu" systems constructed in accordance with my invention.` 'f

Referring 'more particularly tothe drawing, reference numerals .Iv and 2 indicate, respectively,

two pairs of transmitting and reflecting antennae, i

l2, I3, M and l5 match the severalV ycircuit parts at the several frequencies utilized. These matching devices may be of the type disclosed in my copending applications Ser. No. 12,451, filed March 22, 1935; Se-

rial No. 109,658, filed November 7, 1936, or See rialy No. 118,886, filed January 2, 1937, or in Patent No. 1,934,602 to Gilman. Any phase delays introduced by these matching devices are assumed to be equal; when unequal but substantially independent of frequency an additional length of transmission line should be inserted to counteract the error.

This antenna system is adapted unidirectionally to transmit two frequencies F1 and F2, the spacing A between the radiating and reflecting antennae of each pair being such that for frequency F1 the backward radiations from these antennae cancel, while the forward radiations add. This distance A being correct for a cancellation of the backward radiations at F1 would not .ordinarily be correct for the cancellation of the backward radiations at F2, and therefore backward radiations from each pair will take place at frequency F2. However, the spacing B between the two pairs of antennae is made such that the backward radiations from the two pairs at frequency F2 also cancel, thereby assuring that only forward radiation will take place at frequency F2, as well as at frequency F1.

vWhere the radiating and reflecting antennae of a given pair are of the same dimensions, and are fed from a common source, in the simple rectangular manner shown in Fig. 1, the forward radiation will inherently add,` without any special provision for adjustment of phase no matter what value isA chosen for the distance A, while if the distance A between the antennae is made an odd number of quarter wave lengths at frequency F1,` the backward radiations will cancel. On the other hand, if the twopairs of antennae are similar in dimensions and the two spacings A between the radiators and the reflectors are alike, the forward and backward waves from the two pairs, when the antennae are supplied with frequency F2, will be of the same magnitude. The forward waves from the two pairs will thus inherently add at frequency F2 in the same manner as the radiations from the two antennae of either pair inherently added at frequency F1. By making the distance B equal to an odd number of quarter wavelengths at frequency F2, moreover, the backward radiations at frequency F2 from the two pairs of antennae l and 2 will be brought into phase opposition and will also cancel.

Examining the arrangement in more detail, it will be clear that if the speed of propagation of the radiated wave through space from antenna '4 to the antenna 3 is the same as the speed of travel of the wave through the energizing line, the radiated waves from these two antennae at any given forward position will be additive, since the paths of travel of the waves from the common point E to any forward point in space will be, of the same length and the speed of travel through the several paths will be identical. On `the other hand, referring to the backward radiation which results fromreflected waves in the two antennae 3 and d, the wave from antenna 3 will be displaced a distance of 2A with respect tol the backward radiation from antenna 4. Therefore, if the distance A is made an odd number of quarter wavelengths at frequency F1, the distance AA will assure a displacement of an odd number of half wavelengths between the two backward radiations, which will result in the cancellation thereof.

The same facts obtain in connection with the forward and backward radiations of the two antenna pairs I and 2. The forward radiations of the two pairs will always add regardless of di- .mension B. The backward radiations on the other hand can readily be caused to cancel merely by making B an odd number of quarter wavelengths long at frequency F2.

The above description has been based on the assumption that the desired forward radiation which is to be reenforced is horizontal and that the undesired backward radiation to be neutralized is also horizontal and in the opposite direction. It has also been assumed that the transmission line from the common point E to point H of the radiator 3 is longer than the transmission line E to point J of the reflector 4 by an amo-unt equal to the spacial separation of these two antennae 3 and 4. In other words, it has been assumed that the transmission line joining point E to radiator 3 is A units longer than the corresponding transmission line joining point E tothe reflector 4. A similar assumption is, of course, made with respect to the transmission lines feeding antennae 5 and 6 from point D. Furthermore it has also been assumed in an analogous manner that the effective length of the transmission line from common point G to point E is B units longer than the corresponding length of the transmission line from common point G to point D. These assumptions have been made for simplicity in explaining the general principles underlying my invention, since by virtue of these assumptions all the necessary phase relationships for cophasal addition of radiations in the forward direction are inherently satisfied regardless of the choice o-f dimensions A and B.

It is recognized that in practice the desired radiation will not generally be horizontal and therefore the simple relationships above set forth will usually not hold true. For addition of radiation in a desired direction which is qa degrees above the axis of the system, the phase delay of radiator 3 with respect to reflector 4 should be If no phase correcting means were used this would mean that the transmission line from E to radiator 3 should be (A cos qsiNM) units longer than the transmission line from E to refiector 4 instead of being merely A units longer, as in the simple case previously assumed. These same formulae apply of course to the phase delay of radiator 5 with respect to reflector 6 and to the corresponding difference in length between the two feeder lines from point D.

Similarly, if the desired angle of radiation for frequency F2 is 0 degrees above the axis of the system, the phase of antenna pair l with respect to antenna pair 2, will be 2T B cos 0 :t N)

corresponding to a difference of length of units between the transmission line from G to E and the transmission line from G to D.

These alterations in the phases required for correct forward addition at angles other than zero would necessitate corresponding corrections in the distances A and B even if the back radiations were still intended to be neutralized in a s horizontal direction-that is, parallel to the axis of the system. In practice, moreover, it is generally also desired to effect the most perfect cancellation of back radiations at some angle other than zero, which further changes the dimensions A and B. In order to cancel the back radiations of the Waves of frequency F1 at an angle p' with respect to the axis of the system, the phase delay of 'radiator 3 with respect to .reiiector 4 should be equal to (1-3 cfs 0 izN) In order to simultaneously lsatisfy the conditions for forward and back radiation at frequency F1, both spacing A and phase angle P, which represents the phase delay of radiator 3 with respect to reflector 4 and also represents the phase delay of radiator 4 with respect to reiiecto-r E, must be determined by simultaneous solution of the following two equations:

Similarly, both the dimension B and the phase angle Q which is the phase delay of antenna pair I with respect to antenna pair 2, must be determined by simultaneous solution of the two In case the dimensions of the different antennae of the system are not the same, the rules for designing the transmission system which feeds these antennae and for spacing the antennae in the array, must be further modified. The manner of doing this can be most clearly understood from a consideration of Fig. 2, which represents a pair of antennae corresponding to `pair l of Fig. 2, but having two antennae of 1 different lengths. For simplicity it will be assumed that the two antennae 3 and Ll' of Fig. 2 have individual radiation characteristics such that the magnitude of the desired forward component and undesired backward component are equal in magnitude. If these two conditions cannot be simultaneously satisfied, it will be suiiicient if the division of power between the two antennae is so adjusted that the magnitudes of the undesired backward components alone are equal. The length of the radiating antenna 3 is assumed to be L and the length of the reflecting antenna 4' is-assumed to be Z. With respect to the forward radiation it will ordinarily be suiciently accurate to use the rules previously set forth for adjusting the phase `relationships to produce addition in the desired direction. With respect to the undesired back radiation, however, it is necessary to take into account the difference in lengths of the two antennae, since the back radiation is produced by waves traveling backward -along the antennae after reflection from their unterminated open ends.. If the attenuation `of a wave traveling along each of these antennae is suiiiciently high so that only the rst Vreflected wave need be considered, the backwardV radiations may be considered'V as generated at the center points of the' antennae, with phase delays of 1.5 Lg and 1.5 12idegrees, respectively with respect to the phases of the corresponding waves fed to the antennae from the feeder lines. In computing the desired phase delay of the waves fed into antenna 3 with respect tothe waves fed'into antenna 4, therefore, the back radiationr formula previously rset forth should be modified by subtracting the quantity deL-1.502% .l 1

Furthermore, the effective spacing between the two antennae for the purposes of computing back radiation must be considered to be instead of merely A. It will be noted that these corrections are to be used only in those formulae which relate to backward radiation and therefore in solving the simultaneous Formulae 1 and 2, this corrected value must be used in place of the simple value A in Formula 2 alone but notin Formula 1.

'Ihe application of all of the above described rules, formulae and corrections, may best be illustrated by a simple numerical example; Assume that an antenna system substantially as shown in Fig. 1 but having each of its antenna pairs. proportioned as shown in Fig. 2,`is to be designed so as to produce maximum forward' lradiation -at an angle of 14 for-both a frequency F1 whose wave Vlengthis 50 feet and another frequency F2 whose wave length is '75 feet. Assume also that the backward yradiation for both of these frequencies is to be cancelled at the same angle of 14 above the horizontal. Antennae 3 and 5 will each be assumed to have a length L of 175 feet while Vantennae and 6 will each be assumed to havea length l of feet. .Y

Considering separately one single pair of antennae, such as shown in Fig. 2, the phase angle P and spacing A will be found by simultaneously solving the following equations which correspond to Equations 1 and 2 with the corrected value o-f antennae spacing substituting for A in yEquaticn 2. and with the correction factory subtracted from vthe right hand side Simultaneously solving these two equations vwe obtain A=131/ feetiN 51.6 feet The phase angle 93 represents the desired phase delay of the waves fed from transmission line to antenna 3 at point H as compared with waves fed from the transmission line to the antenna 4 at J. The spacing 13% feet represents the distance between point H of antenna 3 and point J of antenna 4. Since the other antenna pair 2 of the system has been assumed to be similar to the antenna pair I, the spacing and phaserelationship of this rear pair 2 will have these same values. With respect to the spacing distance B and the phase angle Q required for producing the desired addition and cancellation of the forward and backward waves at frequency F2, Formulae 3 and 4 may be used without cor- Simultaneously solving these two equations We obtain Thevphase angle 90 represents the phase delay of the waves fed into antenna 3 as compared with the waves fed into antenna 5 or likewise may be considered as representing the phase delay of the waves fed into antenna 4 as compared to the Waves fed into antenna 6. The distance 19.4 feet similarly represents the spacing between any two corresponding points of the two antennae pairs l and 2.

An antenna system dimensioned and phased according to my invention as above described, is capable of radiating waves of two predetermined frequencies F1 and F2 with complete cancellation of the backward radiation in a given direction, as has been fully explained above. In addition to such complete cancellation of backward radiations for the two frequencies F1 and F2, such an antenna system is also capable of radiating other frequencies with substantially no back radiation provided these other frequencies lie in the immediate neighborhood of one of the two predetermined frequencies F1 and F2. The quantitative limits of the proximity required for such substantial cancellation of back radiation cannot readily be defined in accordance with any simple rule. Generally, when the two predetermined frequencies F1 and F2 are close together, all frequencies lying between these two predetermined frequencies, as well as all frequencies lying only slightly outside of these two frequencies, will be transmitted with very little back radiation-i.v e., with a high front to back ratio.

In some bases, moreover, in addition to the two primary predetermined frequencies F1 and F2, there will be one or more additional frequencies lying either between or outside of the two predetermined frequencies at which the back radiation will also be completely cancelled. The occurrence of such an additional frequency of complete cancellation as illustrated, for example, in curve II of Fig. 3, hereafter described, depends upon several factors including the choice of the values of N in the formulae previously set forth. Considering the simplest embodiment first described in conjunction with Fig. 1 wherein the desired directions of additive radiation and neutralization are horizontal and wherein the transmission lines are rectangularly arranged so as to provide inherently additive phase relations in the forward direction, it will be noted that not only the value etc., will provide the desired complete cancellation in the backward direction for frequency Fi. Similarly, considering this same simple system, it will be noted that not only the value but also the values etc., will provide the desired cancellation of backward radiations for frequency F2. According to whether the value etc., is used for dimension A and also according i etc., is used for dimension B, the rate at which the front to back discrimination ratio changes when the frequency of the transmitted wave is l tion is attained. But whether or not any of these additional frequencies will lie in the immediate neighborhood of the primary vpredetermined frequencies F1 and F2, will depend upon the degree of incommensurateness of the spacings A and B.

Curves I and II of Fig. 3, graphically represent the front to back ratio of radiation power for two antenna arrays costructed in accordance with the present invention, the front to back ratios being plotted in decibels as ordinates and the frequencies in megacycles being plotted as abscissae. Curve I represents an array in which the two antennae of each pair are spaced 1A; wavelength apart at the rst predetermined frequency F1, while the spacing between centers of the two pairs is 3X1 wavelength' at a second selected frequency F2. Curve II represents a similar array in which the individual spacing of the antennae of each pair, is again 1/4 wavelength of frequency F1 but in which the centerto-center spacing of the two pairs is wavelengths at frequency F2. In both of these curves frequency F1 is assumed to be 10 megacycles and frequency F2 is assumed to be 13.5 megacycles. The desired directions of addition and cancellation are assumed to be parallel to the axis of the systems and the antennae are assumed to be alike in these curves.

In curve II it is clearly evident that complete cancellation of back radiations will occur, not

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only at megacycles and 13.5 megacycles, but alsov at the additional frequency 8 megacycles. For the antenna array represented by this curve II, therefore, the complete range of frequencies' between 7.5 megacycles and 14.25 megacycles, could be radiated while providing in all cases a front to back ratio of atleast 'l2 decibels. Even if a front to back ratio of at least decibels were considered necessary for satisfactory operation, all frequencies between '7.7 and 10.5 megacycles or between 13.2 andv 13.8 megacycles could be radiated. I

It is thus apparent that the present invention is capable of completely cancelling backward radiation at least two frequencies and sometimeslat one or more additional frequencies within a give useful range. Furthermore, a substantial cancellation of backward radiations may be obtained for all frequencies lying inthe immediate neighborhood of any of the above mentioned frequencies corresponding to complete.

cancellation.

In case a still greater number of points of complete cancellation or a wider range of substantially complete cancellation is desired, the antenna system may, in accordance with my invention, be designed in either of two ways: first, by aA proper choice of the values N in the formulae previously given, the antenna system may be designed so as to present a greater number of points of complete cancellation within the useful range of the antenna, and second, the same general principles of spacing may be extended to more than two groups of antennae. This may, for example, be done by providing two complete antenna systems each containing two groups of two antennae and by selecting the spacing between these two systems with respect to a third predetermined frequency F3. By such an arrangement the spacing of the two antennae within one pair would be selected for cancellation at frequency F, the spacing of the two pairs within one group-of four would be selected for complete cancellation at frequency F2, and

the spacing of the two complete groups of four antennae would be yselected for complete cancellation at frequency F3.

` Although for simplicity of description my invention has been described in conjunction with the' radiation of waves, it may also be used for reception in accordance with the well known reciprocity relation of transmission and reception.

It will be understood that the expression backward direction means merely that direction in which radiation is not desired, or from which reception is to be avoided, at a given frequency. It is preferred to dispose the individual antennae so that they will individually be most efficient in the forward direction. Thus, if V antennae are employed these are preferably arranged with their open ends aimingx generally in the same direction as the forward direction, i. e., the direction toward which radiation is desired or from which reception is desired at the particular frequency. It is possible, however, to aim the V antennae in the opposite sense with respect to the array.

It will be noted also that the forward direction for one frequency need not be the same as for the other frequency but may in fact be completely or substantially opposite.

While I have described particular embodiments of my invention for the purposes of illustration, it should be understood that variations, modiflcations .and adaptations thereof according to one skilled in the art may be made within the' spirit of the invention as set forthin the appended claims.

WhatI claim is:

1`. `A directional antenna system adapted for simultaneous radiant action at two or more different frequencies which comprises two groups of antennae, and means for energizing said. an-v tennae, the individual antennae forming each `group being so spaced apartand energized in such phase relation that the radiant action-thereof in the backward direction cancels at a first fre'- quency and the spacing and phase relation of energization between the two groups of antennae being such that the radiant action of the antennae in the backward direction cancels at a second frequency. y l l 2. A directional antenna system adapted for the simultaneous transmission of two or more' different frequencies which comprises two groups of radiating and reflecting antennae, and means for energizing said antennae, the antennae forming each group being so spaced apart and ener` gized in such phase relation that the backward radiations therefrom cancel at a rst frequency and the spacing and phase relation of energization between the two groups of'antennae being such that the backward radiations from the an`- tennae cancel at a second frequency.

3. A directional antenna system adapted ferent frequencies which comprises two groups 80 K for the simultaneous reception of two or more difof yreceiving and reflecting antennae, and means l' for energizing said antennae, the antennae forming each group being so spaced apart and energizedin such phase relation that the backward waves received thereby cancel at a first frequency and the spacing and phaserelation of energization between the two groups of antennae being such that the backward waves received by ff.

between thetwo pairs of antennae and the phasev relation of energization thereof being suchl that the backward radiations of the antennae cancel at a second frequency, and said distance being a suiiiciently large multiple of the wavelength of said first frequency and said spacing being a sufiiciently large multiple of the wavelength of said second frequency so that the backward radiations from said antennae also cancel at a third frequency in the neighborhood of said first and second frequencies.

5. A directional radio reception system` comprising two pairs of receiving and reflecting antennae, and means for energizing the antennae of said pairs, the two antennae forming each pair being spaced apart a distance and energized in a phase relation such that the backward waves received thereby cancel ata first frequency and the spacing between the two pairs of antennae and the phase relation of energization thereof 6. A directional radio transmission antennal system comprising two pairs of ,radiatingand reecting antennae, and means for energizing the antennae `of, said pairs, the two antennae forming each pair being spaced apart a distance and energized in a phase relation such that the backward radiations therefrom cancel at a first frequency and the spacing between the two pairs of antennae and the phase relation of energization thereof being such that the backward radiations from the antennae cancel at a second frequency, fsaid distance being less than one wavelength of said first frequency and said spacing being less than one wavelength of said second frequency.

7. A directional radio reception antenna system comprising two pairs `of receiving and reecting antennae, and meansfor energizing the antennae of said pairs, the two antennae forming each pair being spaced apart a distance and energized in a phase relation such that the backward waves received thereby cancel lat a rst frequency,

and the spacing between the two pairs of antennae and the phase relation of energization thereof being such that the backward waves received by the antennae cancel at a second frequency, said distance being less than one wave-v length of said first frequency and said spacing `"being less than one wavelength of said second of antennae, and means for energizing said antennae, each including two pairs of antennae, the

individual antennae forming each pair being spaced apart by such a distance and energized in such phase relation that the backward radia- A*tions therefrom cancel at a first frequency and the spacing between the two pairs in each group and the phase energization thereof being such that the backward radiations from the antennae cancel at a second frequency, and the spacing between the two groups being such that the back- Ward radiations from the systemv cancel at a third frequency.

9. A directional antenna system adapted for the simultaneous reception of two or more different frequencies which comprisestwo groups of antennae, and means for energizing said antennae, each including two pairs of antennae, the

individual antennae forming each pair being spaced apart by such a distance and energized in such phase relation that the backward lwaves received thereby cancel at a first frequency and the spacing between the two pairs in each group and the phase energization thereof being such that the backward waves received by the antennae cancel at a second frequency, and the spacing between the two groups being such that the backward waves received by the system cancel at a third frequency. i

`10. A directional antenna system adapted for simulta-neous radiant action at two or more different frequencies which comprises two groups of antennae, the individual antennae forming each group being spaced apart by a distance indicated by the formulae where P represents the phase delay between antennae, A represents the spacing between antennae, y1 represents the wavelength at .one of said different frequencies, N is an integer, c represents the radiation angle above the axis of the antennae and gbl represents the angle above the axis at which radiation cancellation of the backward propagated wave is desired, and the spacing between the two groups of antennae being indicated by the formulae where Q represents the phase delay between the antennae groups, B represents the spacing between said groups, y2 represents the wavelength at the other of said frequencies, N is an integer, represents the radiation angle above the axis of the groups, and 01 represents the angle above the axis of the groups at which radiation cancellation of the backward propagated wave is desired.

ANDREW ALFORD. 

